FIG. 1 illustrates a conventional circuit for detecting faulty diode disclosed in U.S. Pat. No. 6,225,912, issued in May 1, 2001. The circuit for detecting faulty diode is applied in a light-emitting diode (LED) array, which comprises a plurality of LEDs 1, a plurality of comparators 3 and a plurality of transistors 2. Wherein each of the LEDs 1 is associated with a transistor 2 and a comparator 3; each of the LEDs 1 has an anode electrically connected to a driving voltage supply terminal Va (+) and a cathode electrically connected to the negative terminal of the corresponding comparator 3 and the drain of the corresponding transistor 2; each positive terminal of the comparators 3 is connected to a reference voltage input terminal; each gate of the transistors 2 is connected to a turning-on signal input terminal; each source of the transistors 2 is connected to a driving voltage supply terminal Vb (−); and each comparator 3 has a output terminal connected to a faulty signal output terminal.
Assuming that a forward bias of the LED 1 is Vf, the comparators 3 corresponding to the reference voltage input terminal can receive a reference voltage. When the reference voltage is less than Va but greater than (Va−Vf), a process for detecting fault of short circuit can be conducted; otherwise, when the reference voltage is less than (Va−Vf), a process for detecting fault of open circuit can be conducted.
To the process for detecting fault of short circuit, for example, while the LED 1 is operated normally, the voltage applied to the negative terminal of the corresponding comparators 3 is (Va−Vf), and the reference voltage received by the positive terminal of the corresponding comparator 3 is in the range between Va and (Va−Vf). It means that, when the output of the faulty signal output terminal corresponding to the comparator 3 is maintained in a high level (H), which indicates that the LED 1 is operated in a normal situation. On the contrary, while the LED 1 is short, the voltage applied to the negative terminal of the corresponding comparator 3 is Va, and the reference voltage received by the positive terminal of the comparator 3 is less than Va. Thus, the output of the faulty signal output terminal corresponding to the comparator 3 is maintained in a low level (L), which indicates that the LED 1 that acts as a short circuit. That is to say, it can be discriminated that whether the LED 1 is operated in a normal situation or acts as a short circuit in accordance with the voltage level outputted by the faulty signal output terminal corresponding to the comparator 3.
The aforementioned circuit for detecting faulty diode was applied on an optical printer with a plurality of LEDs which functions by the control of either turning on or turning off the LEDs.
FIG. 2 illustrates another conventional circuit for detecting faulty diode disclosed in Taiwanese Patent No. 1299405, issued in Aug. 1, 2008 entitled as “Method and Apparatus for Silent Current Detection”. The circuit for detecting faulty diode comprises a plurality of LEDs 10 connected in series (thereinafter is referred as LED series 10), comparator 30 and transistor 20. Wherein the anode of the LED series 10 is connected to a fixed voltage power supply VLED; the source of the transistor 20 is grounded and the drain of the transistor 20 is connected to the cathode of the LED series 10; the positive terminal of the comparator 30 is applied with a reference voltage Vcc; the negative terminal of the comparator 30 is connected to the source of the transistor 20. In this example, the transistor 20 serves as a driving control circuit and the drain of the transistor 20 is the controlled-output of the driving control circuit.
Similar to FIG. 1, it can be discriminated that whether the LED series 10 is open or short, by the way of investigating the voltage applied to the positive and the negative terminal of the comparator 30.
However, hence the conventional circuit for detecting faulty diode only functions at the moment while the LED associated thereof is either turn on or turn off. Thus, it is not avoidable to prevent the users aware of the LEDs twinkling. Therefore it is not adequate to apply the conventional circuit for detecting faulty diode to a display screen with a plurality of LEDs, which requests the twinkle of the LEDs not being sensed by the users' eyes during operation.